Various types of generators are used for wind turbines for generation of electric power. One commonly used generator is the asynchronous generator which is a high speed generator operating at rotation speeds in the range from 1500 RPM to 3000 RPM. Since the wind turbine rotor of the wind turbine typically rotates with a rotational speed of 20 RPM a gearbox is necessary for increasing the speed from 20 RPM to a least 1500 RPM. Such gearboxes are prone to failures and constitute a high cost of the wind turbine.
Another type of generators used for wind turbines are low-speed direct-drive permanent magnet generators having an operating rotational speed corresponding to the rotational speed of the rotor blades. Therefore, a gearbox is not required when low-speed direct-drive permanent magnet generators are used in a wind turbine. Low-speed direct-drive permanent magnet generators used for wind turbines with 2-3 MW rated power output may have diameters from 5-6 meters and have weights of 5 tons. Generators of this size make transport of the generator to the construction site difficult, especially in case of off-shore wind turbines. Accordingly, replacement of low-speed direct-drive generators in case of failures is a time consuming and costly process.
A first aspect of wind turbines relates to electric power efficiency of generator. Thus, an improvement of the power efficiency may improve the profitability of the investment in wind turbines due to increased power production.
A second aspect of wind turbines relates to the reliability of the generator. Improved reliability of the generator system increases the reliability of the wind turbine, thus increasing the mean-time-between-failure (MTBF) of the wind turbine. The increase of the mean-time-between-failure consequently decreases service costs and increases the power production over time, which is important for the profitability of the investment in wind turbines.
A third aspect of wind turbines relates to the difficulty associated with transporting the components of the wind turbine to the construction site and with replacing components of the wind turbine is case of failures.
WO 00/60719 discloses a low speed generator of the kind being directly coupled to the main shaft of the wind rotor of the windmill. The generator is a stator consisting of a number of stator modules with grain oriented stator sheets that are individual and which may be installed, repaired and dismantled individually and independently of each other. This implies that it is very easy and thereby cheaper to mount the mill, especially at sea, as the stator for the generator can be transported in smaller units, which also makes it easier to assemble the stator in the tower top section. By subsequent repairs and other maintenance of the generator it is not necessary to use large cranes, but it is sufficient to use smaller hoisting devices that may be handled by one or two persons.